When implementing sysexit32, don't let Xen use sysret to return to
userspace.  That results in usermode register state being trashed.
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>